A character string representing the formatted p-value. The function ensures
that the returned string is formatted according to the specified number of decimal places (`my.digit`).
For p-values smaller than 0.001, the function returns "<0.001" to indicate
statistical significance at a high level. This formatting helps in distinguishing
between different levels of statistical significance and can be particularly useful
in reporting the results of statistical tests. The format "0.000" is avoided to
provide a clear indication of very small p-values, enhancing the interpretability
of statistical outputs. For example, a return value of "<0.001" suggests a very
strong evidence against the null hypothesis, whereas a value of "0.045" formatted
with `my.digit=3` suggests a weaker, but still significant, evidence at the 0.05 level.